Antenna Gain Ratings
1. An isotropic antenna is one which has its radiation spread equally in all directions, like a sphere.
2. Of course there is no such antenna but sometimes antenna gains are given as x dbi over an isotropic antenna.
3. A dipole has a gain of 2.1 db over an isotropic antenna.
4. In other words a dipole has a gain of 2.1dbi.
5. A yagi might be said to have a gain of 7dbi which is the same as 4.9dbd if both are in free space, not over reflective ground.
6. In the real world our antenna is mounted over the ground and part of its radiation reflects off the ground which adds or decreases the radiation at various vertical angles creating peaks and nulls depending upon phasing.
7. A high quality ground reflection can double the signal strength at some vertical angles adding 6db to the antenna gain without the ground.